Endocrine secretion of pancreatic islets is regulated by a complex control mechanism driven not only by blob-borne metabolites such as glucose, amino acids, and catecholamines, but also by local paracrine influence. The major pancreatic islet hormones, glucagon, insulin and somatostatin, interact with specific pancreatic cell types (A, B, and D cells, respectively) to modulate the secretory response. Although insulin secretion is predominantly controlled by blood glucose levels, samotostatin inhibits glucose-mediated insulin secretion. In addition to interislet paracrine regulation of insulin secretion, there is evidence to support the existence of insulinotropic factors in the intestine. This incretin concept originates from the observation that food ingestion or enteral glucose administration provoked a greater stimulation of insulin release compared with similar amount of energy (glucose) infused intravenously (Elrick, H., et al., J. Clin. Endocrinol. Metab., 24,1076-1082, 1964; McIntyre, N., et al., J. Clin. Endocrinol. Metab., 25,1317-1324, 1965). Hence, it was postulated that gut-derived signals stimulated by oral nutrient ingestion represent potent insulin secretagogues responsible for the augmentation of insulin release when energy is administered via the gut versus parenteral route (Dupre, J., et al., Diabetes, 15, 555-559, 1966). Although several neurontrasmitters and gut hormones possess incretin-like activity, the considerable evidence from immunization, antagonist, and knockout studies suggest that glucose-dependent insulinotropic polypeptide (GIP) and glucagons-like peptide (GLP)-1 represent the dominant peptides responsible for the majority of nutrient-stimulated insulin secretion. The observation that patient with type 2 diabetes exhibit a significant reduction in the magnitude of meal-stimulated insulin secretion release underlies the interest in determining whether defective incretin release or resistance to incretin action contributes to the pathophysiology of β-cell dysfunction in diabetic subjects.
Glucagon-like peptide-1 (GLP-1) was first identified in 1987 as an incretin hormone, a peptide secreted by the gut upon ingestion of food. GLP1 is secreted by the L-cell of the intestine after being proteolytically processed from the 160amino acid precursor protein, preproglucagon. Cleavage of preproglucagon first yields GLP-1, a 37 amino acid peptide, GLP-1(1-37)OH, that is poorly active. A subsequent cleavage at the 7-position yields biologically active GLP-1(7-37)OH. Approximately 80% of GLP-1(7-37)OH that is synthesized is amidated at the C-terminal after removal of the terminal glycine residue in the L-cell. The biological effects and metabolic turnover of the free acid GLP-1(7-37)OH and the amide, GLP-1(7-37)NH2, are indistinguishable.
GLP-1 is known to stimulate insulin secretion causing glucose uptake by cells which decrease serum glucose levels (Mojsov, S., et al., J. Clin.Invest., 79, 616-619, 1987; Kreymann, B., et al., Lancet ii,1300-1304, 1987; Orskov, C.,et al., Endocrinology, 123, 2009-2013, 1988). Acute intracerebroventricular injection of GLP-1 or GLP-1 receptor agonists produces transient reduction in food intake (Turton M.D., et al., Nature, 379, 60-72, 1996), whereas more prolonged intracerebroventricular or parenteral GLP-1 receptor agonists administration is associated with weight loss in some studies(Meeran, K., et al., Endocrinology, 140, 244-250, 1999; Davies, H. R. Jr., Obes. Res., 6, 147-156, 1998; Szayna, M., et al., Endocrinology, 141, 1936-1941, 2000; Larsen, P. J., et al., Diabetes, 50, 2530-2539, 2001). Numerous GLP-1 analogs demonstrating insulinotropic action are know in the art. These analogs include, for example, GLP-1(7-36), Gln9-GLP-1(7-37), D-Gln9-GLP-1(7-37), acetyl- Lys9-GLP-1(7-37), Thr16-Lys18-GLP-1(7-37) and Lys18-GLP-1(7-37). Derivatives of GLP-1 include, for example, acid addition salts, carboxylate salts, lower alkyl esters, and amides (WO91/11457; EP0733644; U.S. Pat. No. 5,512,549).
The majority of GLP-1 action delineated in preclinical experiments has also been demonstrated in human studies. Infusion of GLP-1(7-36)NH2 into normal human subjects stimulated insulin secretion, significantly reduced blood glucose in the fasting state after glucose loading or food ingestion (Orskov, C., et al., Diabetes, 42, 658-661, 1993; Qualmann, C., et al., Acta.Diabetol., 32, 13-16, 1995).
GLP-1 based peptides hold great promise as alternatives to insulin therapy for patients with diabetes who have failed on treatment with sulfonylureas (Nauck, M.A. et al., Diabetes Care, 21, 1925-1931, 1998). GLP-1 stimulates insulin secretion, but only during period of hyperglycemia. The safety of GLP-1 compared to insulin is enhanced by this property of GLP-1 and by the observation that the amount of insulin secreted is proportional to the magnitude of the hyperglycemia. In addition, GLP-1 therapy will result in pancreatic release of insulin and first-pass insulin action in the liver. These results in lower circulating levels of insulin in the periphery compared to subcutaneous insulin injections. GLP-1 slows gastric emptying which is desirable in that it spreads nutrient absorption over a longer period of time, decreasing the postprandial glucose peak. Several reports may suggest that GLP-1 can enhance insulin sensitivity in peripheral tissues such muscle, liver, and fat. Finally, GLP-1 has been shown to be a potential regulator of appetite.
The therapeutic potential for GLP-1 and its analogs is further increased if one considers its use in patients with type 1 diabetes. A number studies have demonstrated the effectiveness of native GLP-1 in the treatment of insulin dependent diabetes mellitus (IDDM).
Similar to non-insulin dependent diabetes mellitus (NIDDM) patients, GLP-1 is effective in reducing fasting hyperglycemia through its glucagonostatic properties. Additional studies have indicated that GLP-1 also reduces postprandial glycemic excursion in IDDM, most likely through a delaying in gastric emptying. These observations suggest that GLP-1 may be useful as a treatment for IDDM as well as for NIDDM.
However, the biologic half-life of native GLP-1 molecules which are effected by the activity of dipeptidyl-peptidase IV (DPP IV) is quite short. For example, the biological half-life of GLP-1(7-37)OH is a mere 3 to 5 minutes (U.S. Pat. No. 5,118,666). Sustained lowering of blood glucose concentration is only observed with continuous infusion, as demonstrated in studies in which GLP-1 was administered by intravenous infusion over 24 hr time course (Larsen, J. et al. Diabetes Care, 24, 1416-1421, 2001). The enzyme DPP IV, a serine protease that preferentially hydrolyzed peptides after a penultimate NH2-terminal proline (Xaa-Pro-) or alanine (Xaa-Ala-) (Mentlein, R., Regul. Pept., 85, 9-25, 1999), has been shown to rapidly metabolize GLP-1 in vitro. Therefore extended- action GLP-1 based peptides that are resistant to DPP IV may have great therapeutic potential for treatment of diabetes mellitus.